Data communication over wireless lines has come to be performed frequently. With an existing so-called third generation mobile communication system or 3.5th generation mobile communication system as well, data communication has come to be widely used with application use such as e-mail, web browsing, or the like. In recent years, service providing of a so-called 3.9th generation mobile communication system higher than those existing systems has been started, and also service of a further higher speed fourth generation mobile communication system is also planned to be started. Also, widespread use of smartphones has led to increase in wireless data communication. Under such a situation, data communication over wireless lines is expected to increase more and more in the future, and the importance thereof is increasing more and more.
In general, with wireless communication, as compared to wired communication, signal error readily occurs due to interference of radio waves or the like. Therefore, with wireless communication, a technique to enhance communication reliability has been employed, such as ARQ (Automatic Repeat reQuest) or FEC (Forward Error Correction).
ARQ is a technique to enhance reliability of wireless communication by performing retransmission in the event that a wireless frame is not correctly received. First, a transmission device transmits a wireless frame. In the event of having correctly received the wireless frame (in the event of having received the wireless frame and also having no reception error in the received wireless frames), a reception device responds with an ACK (ACKnowledge: reception acknowledgement) signal to the transmission device. In the event of having received no ACK signal from the reception device, the transmission device performs retransmission of the wireless frame after waiting for timeout.
The transmission device repeats this operation until the transmission device receives an ACK signal. The upper limit of the number of times of repeating this operation is called “the maximum number of times of retransmission”, which is one parameter of retransmission control. In the event of having received an ACK signal within the maximum number of times of retransmission, this means to have succeeded at transmission of the wireless frame using retransmission control. Also, in the event of having received no ACK signal after performing retransmission by the maximum number of times of retransmission, the transmission device ends retransmission control. This case means to have failed in transmission of the wireless frame. The greater the maximum number of times of retransmission is, the higher a probability to succeed in transmission of the wireless frame increases, and the smaller the maximum number of times of retransmission is, the lower a probability to succeed in transmission of the wireless frame decreases. Even in the event that it is difficult to correctly receive the wireless frame due to deterioration in wireless quality, communication reliability with a wireless line is secured by employing ARQ.
On the other hand, FEC is a technique to transmit redundant data generated from transmission data by loading on the wireless frame beforehand, and to restore, in the event that there is a transmission error, data including an error from the redundant data. The redundant data may be loaded on the same wireless frame as the transmission data, or may be loaded on a frame different from the transmission data. Also, there may be a plurality of redundant data, and in this case, the redundant data is distributed and loaded on multiple frames.
With FEC, basically, retransmission such as ARQ does not to be performed (however, there is an exception in the event of combining FEC and ARQ, which will be described later). With FEC, redundant data is used, and accordingly, though there is an aspect where efficiency in data transmission decreases, retransmission does not have to be performed, whereby data transmission may effectively be performed instead in the event that wireless quality is deteriorated, or the like. That is to say, even in the event that frames are readily damaged due to deterioration in wireless quality, reliability of communication with a wireless line is secured by employing FEC.
ARQ and FEC both have in common being techniques to enhance reliability of communication. Therefore, frequently, just one thereof is employed. In general, ARQ is adapted to a network with small transmission delay, and FEC is adapted to a network with great transmission delay. However, these are not necessarily selective techniques, and both may also be employed together. Examples of a technique to employ both together include HARQ (Hybrid ARQ).
Also, there has been popular a technique to carry out ARQ and FEC by switching at a wireless layer according to RTT (Round Trip Time: round-trip delay time) at an upper layer which is a wireless layer, as the related art.
Japanese Laid-open Patent Publication Nos. 2009-290393 and 2003-179580 are examples of the related art of the present technology.